How to pass function scope (not context)? - javascript

Can you please help me to find the error in the code? I need to pass one method's scope to another method.
var obj = {
doIt: function() {
var a = 10;
this.doThat.call(this);
},
doThat: function() {
console.log(a);
}
};
obj.doIt();

If you want to access a in the scope of context, you can try this.
var obj = {
doIt: function() {
this.a = 10;
this.doThat.call(this);
},
doThat: function() {
console.log(this.a);
}
};
obj.doIt();

You do pass the "context" (aka this) around. a however, is not part of the current context, but of the current scope. So you either move a into the context:
var obj = {
doIt: function() {
this.a = 10;
this.doThat();
},
doThat: function() {
console.log(this.a);
}
};
Or you directly pass the value of a into doThat:
//...
this.doThat(a);
doThat(a) { /*...*/ }

You need to understand that there is difference between context and scope. scope is going to be something which will dictate which variables are accessible from a particular site whereas context is what will be the value of this at particular site or inside a function.
so if you want to use value of a in function doThat you need to bind value of a with this which is context in function doIt like: this.a = 10; and then access it using console.log(this.a) inside function doThat

Related

How to pass down function with "this" keyword [duplicate]

I have a question concerning how the "this" pointer is treated in a nested function scenario.
Say I insert this following sample code into a web page. I get an error when I call the nested function "doSomeEffects()". I checked in Firebug and it indicates that when I am in that nested function, the "this" pointer is actually pointing to the global "window" object - which I did not expect. I must not be understanding something correctly because I thought since I declared the nested function within a function of the object, it should have "local" scope in relation to the function (i.e. the "this" pointer would be referring to the object itself like how it is in my first "if" statement).
Any pointers (no pun intended) would be appreciated.
var std_obj = {
options : { rows: 0, cols: 0 },
activeEffect : "none",
displayMe : function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect=="fade") { }
var doSomeEffects = function() {
// the 'this' pointer is referring to the window obj, why?
if (this.activeEffect=="fade") { }
}
doSomeEffects();
}
};
std_obj.displayMe();
In JavaScript the this object is really based on how you make your function calls.
In general there are three ways to setup the this object:
someThing.someFunction(arg1, arg2, argN)
someFunction.call(someThing, arg1, arg2, argN)
someFunction.apply(someThing, [arg1, arg2, argN])
In all of the above examples the this object will be someThing.
Calling a function without a leading parent object will generally get you the global object which in most browsers means the window object.
Since this appears to be among the most upvoted questions of its kind, let me add, after all these years, the ES6 solution using arrow functions:
var std_obj = {
...
displayMe() {
...
var doSomeEffects = () => {
^^^^^^^ ARROW FUNCTION
// In an arrow function, the 'this' pointer is interpreted lexically,
// so it will refer to the object as desired.
if (this.activeEffect=="fade") { }
};
...
}
};
this is not part of the closure scope, it can be thought of as an additional parameter to the function that is bound at the call site. If the method is not called as a method then the global object is passed as this. In the browser, the global object is identical to window. For example, consider the following funciton,
function someFunction() {
}
and the following object,
var obj = { someFunction: someFunction };
If you call the function using method syntax such as,
obj.someFunciton();
then this is bound to obj.
If you call someFunction() directly, such as,
someFunction();
then this is bound to the global object, that is window.
The most common work around is to capture this into the closure such as,
displayMe : function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect=="fade") { }
var that = this;
var doSomeEffects = function() {
// the 'this' pointer is referring to global
// that, however, refers to the outscope this
if (that.activeEffect=="fade") { }
}
doSomeEffects();
}
To understand this question , try to get the output for the following snippet
var myObject = {
foo: "bar",
func: function() {
var self = this;
console.log("outer func: this.foo = " + this.foo);
console.log("outer func: self.foo = " + self.foo);
(function() {
console.log("inner func: this.foo = " + this.foo);
console.log("inner func: self.foo = " + self.foo);
}());
}
};
myObject.func();
The above code will output the following to the console:
outer func: this.foo = bar
outer func: self.foo = bar
inner func: this.foo = undefined
inner func: self.foo = bar
In the outer function, both this and self refer to myObject and therefore both can properly reference and access foo.
In the inner function, though, this no longer refers to myObject. As a result, this.foo is undefined in the inner function, whereas the reference to the local variable self remains in scope and is accessible there. (Prior to ECMA 5, this in the inner function would refer to the global window object; whereas, as of ECMA 5, this in the inner function would be undefined.)
There's a difference between enclosure variables and "this". "this" is actually defined by the invoker of the function, while explicit variables remain intact inside the function declaration block known as the enclosure. See the example below:
function myFirstObject(){
var _this = this;
this.name = "myFirstObject";
this.getName = function(){
console.log("_this.name = " + _this.name + " this.name = " + this.name);
}
}
function mySecondObject(){
var _this = this;
this.name = "mySecondObject";
var firstObject = new myFirstObject();
this.getName = firstObject.getName
}
var secondObject = new mySecondObject();
secondObject.getName();
you can try it out here:
http://jsfiddle.net/kSTBy/
What's happening in your function is "doSomeEffects()", is being called explicitly, this means context or the "this" of the function is the window. if "doSomeEffects" was a prototype method e.g. this.doSomeEffects on say "myObject", then myObject.doSomeEffects() would cause "this" to be "myObject".
As explained by Kyle, you could use call or apply to specify this within the function:
Here is that concept applied to your code:
var std_obj = {
options: {
rows: 0,
cols: 0
},
activeEffect: "none",
displayMe: function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect == "fade") {}
var doSomeEffects = function() {
// the 'this' pointer is referring to the window obj, why?
if (this.activeEffect == "fade") {}
}
doSomeEffects.apply(this,[]);
}
};
std_obj.displayMe();
JsFiddle
Since it wasn't mentioned I will mention that using .bind() is a solution -
doSomeEffects=doSomeEffect.bind(this);
doSomeEffects();
}
};
std_obj.displayMe();
Here is a more simple example -
bad = {
name:'NAME',
z : function() {
function x() { console.log(this.name); };
x()
}
};
bad.z() // prints 'undefined'
good = {
name:'NAME',
z : function() {
function x() { console.log(this.name); };
x=x.bind(this);
x();
}
};
good.z() // prints 'NAME'
It is true that using an arrow function => looks slicker and is easy for the programmer. However, it should be kept in mind that a lexical scope is likely to require more work in terms of processing and memory to setup and maintain that lexical scope, compared to simply associating a function's this with a pointer via .bind().
Part of the benefit of developing classes in JS was to provide a method to make this more reliably present and available, to pivot away from functional programming and lexical scopes, and thus reduce overhead.
From MDN
Performance considerations
It is unwise to unnecessarily create functions within other functions if closures are not needed for a particular task, as it will negatively affect script performance both in terms of processing speed and memory consumption.
It's because "this" refers to the self object / local function.
var std_obj = {
options : { rows: 0, cols: 0 },
activeEffect : "none",
displayMe : function() {
if (this.activeEffect=="fade") { }
let This = this; // 'this' here is for the std_obj scope. Create a reference to 'this' if you want to use it elsewhere.
var doSomeEffects = function() {
// 'this' here refers to the doSomeEffects scope. If you don't want "this," you can still use "this" of the std_obj scope.
if (This.activeEffect=="fade") { }
}
doSomeEffects();
}
};
std_obj.displayMe();
I also got a warning "Potentially invalid reference access to a class field via this"
class MyListItem {
constructor(labelPrm) {
this._flagActive = false;
this._myLabel = labelPrm;
labelPrm.addEventListener('click', ()=>{ this.onDropdownListsElementClick();}, false);
}
get myLabel() {
return this._myLabel
}
get flagActive(){
return this._flagActive;
}
onDropdownListsElementClick(){console.log("Now'this'refers to the MyListItem itself")}}//end of the class

`this` inside inner functions [duplicate]

I have a question concerning how the "this" pointer is treated in a nested function scenario.
Say I insert this following sample code into a web page. I get an error when I call the nested function "doSomeEffects()". I checked in Firebug and it indicates that when I am in that nested function, the "this" pointer is actually pointing to the global "window" object - which I did not expect. I must not be understanding something correctly because I thought since I declared the nested function within a function of the object, it should have "local" scope in relation to the function (i.e. the "this" pointer would be referring to the object itself like how it is in my first "if" statement).
Any pointers (no pun intended) would be appreciated.
var std_obj = {
options : { rows: 0, cols: 0 },
activeEffect : "none",
displayMe : function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect=="fade") { }
var doSomeEffects = function() {
// the 'this' pointer is referring to the window obj, why?
if (this.activeEffect=="fade") { }
}
doSomeEffects();
}
};
std_obj.displayMe();
In JavaScript the this object is really based on how you make your function calls.
In general there are three ways to setup the this object:
someThing.someFunction(arg1, arg2, argN)
someFunction.call(someThing, arg1, arg2, argN)
someFunction.apply(someThing, [arg1, arg2, argN])
In all of the above examples the this object will be someThing.
Calling a function without a leading parent object will generally get you the global object which in most browsers means the window object.
Since this appears to be among the most upvoted questions of its kind, let me add, after all these years, the ES6 solution using arrow functions:
var std_obj = {
...
displayMe() {
...
var doSomeEffects = () => {
^^^^^^^ ARROW FUNCTION
// In an arrow function, the 'this' pointer is interpreted lexically,
// so it will refer to the object as desired.
if (this.activeEffect=="fade") { }
};
...
}
};
this is not part of the closure scope, it can be thought of as an additional parameter to the function that is bound at the call site. If the method is not called as a method then the global object is passed as this. In the browser, the global object is identical to window. For example, consider the following funciton,
function someFunction() {
}
and the following object,
var obj = { someFunction: someFunction };
If you call the function using method syntax such as,
obj.someFunciton();
then this is bound to obj.
If you call someFunction() directly, such as,
someFunction();
then this is bound to the global object, that is window.
The most common work around is to capture this into the closure such as,
displayMe : function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect=="fade") { }
var that = this;
var doSomeEffects = function() {
// the 'this' pointer is referring to global
// that, however, refers to the outscope this
if (that.activeEffect=="fade") { }
}
doSomeEffects();
}
To understand this question , try to get the output for the following snippet
var myObject = {
foo: "bar",
func: function() {
var self = this;
console.log("outer func: this.foo = " + this.foo);
console.log("outer func: self.foo = " + self.foo);
(function() {
console.log("inner func: this.foo = " + this.foo);
console.log("inner func: self.foo = " + self.foo);
}());
}
};
myObject.func();
The above code will output the following to the console:
outer func: this.foo = bar
outer func: self.foo = bar
inner func: this.foo = undefined
inner func: self.foo = bar
In the outer function, both this and self refer to myObject and therefore both can properly reference and access foo.
In the inner function, though, this no longer refers to myObject. As a result, this.foo is undefined in the inner function, whereas the reference to the local variable self remains in scope and is accessible there. (Prior to ECMA 5, this in the inner function would refer to the global window object; whereas, as of ECMA 5, this in the inner function would be undefined.)
There's a difference between enclosure variables and "this". "this" is actually defined by the invoker of the function, while explicit variables remain intact inside the function declaration block known as the enclosure. See the example below:
function myFirstObject(){
var _this = this;
this.name = "myFirstObject";
this.getName = function(){
console.log("_this.name = " + _this.name + " this.name = " + this.name);
}
}
function mySecondObject(){
var _this = this;
this.name = "mySecondObject";
var firstObject = new myFirstObject();
this.getName = firstObject.getName
}
var secondObject = new mySecondObject();
secondObject.getName();
you can try it out here:
http://jsfiddle.net/kSTBy/
What's happening in your function is "doSomeEffects()", is being called explicitly, this means context or the "this" of the function is the window. if "doSomeEffects" was a prototype method e.g. this.doSomeEffects on say "myObject", then myObject.doSomeEffects() would cause "this" to be "myObject".
As explained by Kyle, you could use call or apply to specify this within the function:
Here is that concept applied to your code:
var std_obj = {
options: {
rows: 0,
cols: 0
},
activeEffect: "none",
displayMe: function() {
// the 'this' pointer is referring to the std_obj
if (this.activeEffect == "fade") {}
var doSomeEffects = function() {
// the 'this' pointer is referring to the window obj, why?
if (this.activeEffect == "fade") {}
}
doSomeEffects.apply(this,[]);
}
};
std_obj.displayMe();
JsFiddle
Since it wasn't mentioned I will mention that using .bind() is a solution -
doSomeEffects=doSomeEffect.bind(this);
doSomeEffects();
}
};
std_obj.displayMe();
Here is a more simple example -
bad = {
name:'NAME',
z : function() {
function x() { console.log(this.name); };
x()
}
};
bad.z() // prints 'undefined'
good = {
name:'NAME',
z : function() {
function x() { console.log(this.name); };
x=x.bind(this);
x();
}
};
good.z() // prints 'NAME'
It is true that using an arrow function => looks slicker and is easy for the programmer. However, it should be kept in mind that a lexical scope is likely to require more work in terms of processing and memory to setup and maintain that lexical scope, compared to simply associating a function's this with a pointer via .bind().
Part of the benefit of developing classes in JS was to provide a method to make this more reliably present and available, to pivot away from functional programming and lexical scopes, and thus reduce overhead.
From MDN
Performance considerations
It is unwise to unnecessarily create functions within other functions if closures are not needed for a particular task, as it will negatively affect script performance both in terms of processing speed and memory consumption.
It's because "this" refers to the self object / local function.
var std_obj = {
options : { rows: 0, cols: 0 },
activeEffect : "none",
displayMe : function() {
if (this.activeEffect=="fade") { }
let This = this; // 'this' here is for the std_obj scope. Create a reference to 'this' if you want to use it elsewhere.
var doSomeEffects = function() {
// 'this' here refers to the doSomeEffects scope. If you don't want "this," you can still use "this" of the std_obj scope.
if (This.activeEffect=="fade") { }
}
doSomeEffects();
}
};
std_obj.displayMe();
I also got a warning "Potentially invalid reference access to a class field via this"
class MyListItem {
constructor(labelPrm) {
this._flagActive = false;
this._myLabel = labelPrm;
labelPrm.addEventListener('click', ()=>{ this.onDropdownListsElementClick();}, false);
}
get myLabel() {
return this._myLabel
}
get flagActive(){
return this._flagActive;
}
onDropdownListsElementClick(){console.log("Now'this'refers to the MyListItem itself")}}//end of the class

How to keep 'this' in a class from within a javascript closure

I have read this answer and IIFE but I can't seem to find the correct solution to my problem.
I have a simple class here:
define(['jquery'], function($) {
// Need 'self' because someCallback() is being called with .call() and 'this' changes
var self;
function Foo(number) {
self = this;
this.someNumber = number;
}
Foo.prototype = {
someCallback: function () {
//Use self because 'this' changes to a DOM element
var num = self.someNumber;
//Do something with the num
return num * 2;
}
};
return Foo;
});
and someCallBack() is being called by a jQuery plugin using .call(). Because of this, the context changed, hence the use of the self variable.
However, this is wrong because:
define(['foo'], function(Foo) {
describe('context question', function () {
var foo1 = new Foo(1);
var foo2 = new Foo(2);
it('"this" should work', function () {
var call1 = foo1.someCallback.call(this); // 4
var call2 = foo2.someCallback.call(this); // 4
expect(call2).toBe(4); // Only works because it is 'new' last
expect(call1).toBe(2); // Fails because 'self' is taken from foo2
});
});
});
How exactly should I wrap the self variable to make this code work?
You could probably just use the revealing module pattern and declare it as a "global" variable (local to the module):
define(['jquery'], function($) {
var someNumber;
function Foo(number) {
someNumber = number;
}
Foo.prototype = {
someCallback: function () {
return someNumber * 2;
}
};
return Foo;
});
Two ways of calling an object method which stores its own this value include
Define the method as a nested function which references its this value in a closure which stores this value in a variable. The function defined could be anonymous or declared with a name but must be evaluated each time a class instance is created, so as to create a new Function object capturing different values of self in function scope.
Take a statically defined function object and bind its this value using bind. Bind creates a new wrapper function object each time it is called.
The first method looks like (without Jquery or Jasmine):
function Foo(number)
{ var self = this;
this.num = number;
this.someCallback = function() // method with new Foo object stored as self in function scope
{ // something with num:
return self.num * 2;
}
}
and the second method could look like
function Foo(number)
{ this.num = number
this.someCallback = this.someCallback.bind(this); // bind prototypical method as local method.
}
Foo.prototype = {
someCallback: function () {
// this value is bound by constructor;
//Do something with the num
return this.num * 2;
}
};

Is it possible to pass execution context of the immediately invoked function expression

Consider the following code:
(function() {
var a = 5;
var someFunc = function() { ... };
function anotherFunc() {
...
};
})();
window.myGlobalObj = {
init: function() {
// and somehow here I want to access to the IIFE context
}
};
I want to have the execution context of IIFE in my global object. I do have access to function expression and object itself so I can pass or modify something to make it work (and no, I can't rewrite everything inside the object or function).
Is it even possible?
The only way I see how that's poosible is by using eval to simulate dynamic scopes. Do this (note that the IIFE must be placed after the global object):
window.myGlobalObj = {
init: function() {
// and somehow here I want to access to the IIFE context
}
};
(function() {
var a = 5;
var someFunc = function() { ... };
function anotherFunc() {
...
};
eval("(" + String(window.myGlobalObj.init) + ")").call(window.myGlobalObj);
})();
Here's a reference as on how to use dynamic scopes: Is it possible to achieve dynamic scoping in JavaScript without resorting to eval?
Edit: I've included an example to demonstrate the power of using dynamic scopes in JavaScript. You can play with the fiddle too.
var o = {
init: function () {
alert(a + b === this.x); // alerts true
},
x: 5
};
(function () {
var a = 2;
var b = 3;
eval("(" + String(o.init) + ")").call(o);
}());
The "contents" of your IIFE, i.e., a, someFunc, etc., are local to that function scope, so you can only access them within that scope. But you can assign window.myGlobalObj inside the IIFE:
(function() {
var a = 5;
var someFunc = function() { ... };
function anotherFunc() {
...
};
window.myGlobalObj = {
init: function() {
// and somehow here I want to access to the IIFE context
}
};
})();
Then the init function will have access to those variables since they are in its containing scope.
EDIT: if you can't move the definition of myGlobalObj into the IIFE the only thing I can think of is to use the IIFE to create a second global object that you access from myGlobalObj:
(function() {
var a = 5;
var someFunc = function() { ... };
function anotherFunc() {
...
};
// create a global object that reveals only the parts that you want
// to be public
window.mySecondObject = {
someFunc : someFunc,
anotherFunc : anotherFunc
};
})();
window.myGlobalObj = {
init: function() {
window.mySecondObject.someFunc();
}
};
No. It is not possible. The context you want to access is called closure and can be accessed only within the function (in your case, the anonymous function (IIFE how you call it)). For more about closures follow the excellent Douglas Crockfords The Javascript programming language video tutorial.
You will have to place those attributes to some shared object.

Passing Scope from one function to another

I have an object litrel that has 2 anonymous functions defined as such:
obj = {
funcA : function(){
var a = 'a';
},
funcB : function(){
// but how do you access the scope in 'funcA' to access variable 'a'?
console.log(a)
}
}
I do not want to pass any variables just the scope of 'funcA' - thoughts?
JavaScript scope doesn't work like that because it has (only) function scope (except when using the let keyword).
What you could do is...
obj = {
a: 0,
funcA: function() {
this.a = 'a';
},
funcB: function() {
console.log(this.a);
}
};
jsFiddle.
You can use the a closure with the module pattern to get access to variables in an outer scope. More than one function can accesses the variable:
var obj = (function() {
var a;
return {
setA: function(value) {
a = value;
return a;
},
getA: function(value) {
return a;
},
multiplyA: function(value) {
a = a * value;
return a;
}
};
}());
alert( obj.setA(5) ); // 5
alert( obj.multiplyA(2) ); // 10
alert( obj.getA() ); // 10
See Douglas Crockford's article on Private Members in JavaScript. In this way, functions can share a common variable object on their scope chain. You still can't reference it or pass it to another function, but you might be able to get the functionality you want.
Does this code do what you want? If not, I'm really curious as to why.
obj = {
funcA : function(){
this.a = 'a';
alert(this.a);
},
funcB : function(){
// but how do you access the scope in 'funcA' to access variable 'a'?
alert(this.a);
}
}
obj.funcA();
obj.funcB();
What is the difference between accessing the actual scope of a separate function and setting members on the parent object as I have done?
Also, since there isn't a way of passing scope, are you just going to use something similar to this code instead?

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